The present invention relates to a thermal transfer recording medium capable of multiple printing (which is hereinafter sometimes referred to simply as a "thermal transfer recording medium"). More particularly, the present invention relates to a thermal transfer recording medium that is capable of providing a sharp, ghost-free image through many cycles of printing operation.
Various versions of thermal transfer recording media capable of multiple printing have heretofore been reported and they include: a thermal transfer recording medium of a "cohesive failure" type that comprises a comparatively thick, single ink layer formed on a support with a resin layer interposed, and which performs multiple printing with only a part of the ink layer being used for each cycle of transfer [see JP-A-57-36698 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")]; a thermal transfer recording medium of an "exudation" type that uses an ink layer having a heat-fusible ink in a high-molecular weight porous material and which performs multiple printing with the heat-fusible ink exudating in small portions from the porous material (see JP-A-54-68235); and a thermal transfer recording medium of a "fence" type that uses an ink layer having a fine particulate filler and a heat-fusible ink and which performs multiple printing with the amount of ink transfer per impression being controlled by the fine particulate filter (see JP-A-57-160691). These prior art thermal transfer recording media, however, have had the disadvantage that in second and subsequent impressions, "ghost" characterized by uneven densities in image areas, interruption of characters and other phenomena due to previous impressions occurs on the surface of prints.
It is not completely clear why "ghost" occurs but according to the studies conducted by the present inventor, the following explanation may be postulated: in a thermal transfer recording medium of a "cohesive failure" type, the ink layer has fine asperities formed on the surface following the first impression and these asperities create unevenness in platen pressure in the next impression, thereby causing "ghost". The ghost occurring in thermal transfer recording media of "exudation" and "fence" types could be explained in the same way.
Another problem with the prior art thermal transfer recording media designed for multiple printing is that the ink layer is so susceptible to temperature changes that if they are used in a hot environment, an increased amount of ink will be transferred per impression, thus reducing the number of printing cycles that can be accomplished with the medium. If, on the other hand, the medium is used in a cold environment, the amount of ink transfer per impression will decrease to produce very low densities in prints.